1. Field of the Invention
The invention relates to a new liquid crystal compound that is useful as a material for use in a liquid crystal display device, and a liquid crystal composition including this compound. More specifically, the invention relates to a new liquid crystal compound that has a small viscosity and an excellent compatibility with other liquid crystal compounds, and further has a suitable optical anisotropy and dielectric anisotropy, and that gives a steep electro-optic characteristics when the compound is used for a liquid crystal display device, and relates to a liquid crystal composition that includes this compound and to a liquid crystal display device that contains this liquid crystal composition.
2. Related Art
A display device containing a liquid crystal compound has been widely used for displays of a computer, a television set and so forth. In this display device, the optical anisotropy, the dielectric anisotropy and so forth of the liquid crystal compound are utilized.
A liquid crystal phase includes a nematic liquid crystal phase, a smectic liquid crystal phase, a cholestric liquid crystal phase, and the nematic phase is most widely applied. An operating mode includes modes such as DS (dynamic scattering), DAP (deformation of aligned phases), GH (guest-host), TN (twisted nematic), STN (super twisted nematic), TFT (a thin film transistor), VA (vertical alignment), IPS (in-plane switching) and PSA (polymer sustained alignment).
A liquid crystal compound used for these operating modes is required to have a liquid crystal phase in a wide temperature range including room temperature, a sufficient stability under conditions that the display device is used, and also sufficient characteristics for driving the display device. However, no single liquid crystal compounds that satisfy these conditions have been found until now.
The actual situation is that a liquid crystal composition is prepared by mixing from several to several tens of liquid crystal compounds in order to satisfy the required characteristics. It is required that the liquid crystal composition is stable to moisture, light, heat and air, which are normally present under conditions that the display is used, and to an electric field or electromagnetic radiation, and also is stable chemically to a compound that will be mixed. It is required that the liquid crystal composition has suitable values of characteristics such as optical anisotropy (Δn) and dielectric anisotropy (Δ∈), depending on the operating mode or the shape of the display device. Furthermore, it is important that each component in the liquid crystal composition has an excellent solubility in each other.
It is desirable for an excellent liquid crystal display that the product (Δn×d) of the cell thickness of the liquid crystal display device and the optical anisotropy (Δn) of the liquid crystal composition used is constant. See E. Jakeman, et al., Phys. Lett., 39A. 69 (1972). The response speed of the liquid crystal display device is inversely proportional to the square of the cell thickness. Accordingly, a liquid crystal composition having a large optical anisotropy should be used in order to produce a liquid crystal display device that is able to respond at high speed and thus can be applied to the display of moving images and so forth. A variety of liquid crystal compounds having a large optical anisotropy have been synthesized until now. Since such a compound having a large optical anisotropy generally has a highly conjugated molecular structure, the compound has a tendency to have a poor compatibility with the other liquid crystal compounds, and thus it is difficult to use the compound as a component of a liquid crystal composition having excellent electrical characteristics. Further, in a liquid crystal compound used as a component of a liquid crystal composition, which is required to be a high insulation (specific resistance), for using a liquid crystal display device with a thin film transistor type, a high stability is required.
In the operating mode described above, the VA mode, the PSA mode and so forth utilize homeotropic orientation of liquid crystal molecules, and it is known that a limited viewing angle, which is a disadvantage of a conventional display mode such as the TN mode and the STN mode, can be improved by means of these modes.
A liquid crystal compound having a trans-monofluoroethylene group as a bonding group has conventionally been studied as a component of a liquid crystal composition usable for these liquid crystal display devices.
The patent document No. 1 discloses the compounds (S-1) and (S-2), and these compounds have features that are small in viscosity and stable to light. However, a compound having negative dielectric anisotropy is not disclosed.
R and R′ are an alkyl group.
The patent document No. 2 discloses the compounds (S-3) and (S-4) having negative dielectric anisotropy and having an ethylene group as a bonding group. However, a compound in which fluorine is introduced in the ethylene group is not disclosed.
R is an alkyl group.
3. References    No. 1: JP H05-070382 A (1993).    No. 2: JP 2002-193852 A.